Some years since, during Mr. Maclure's geological survey of the United States, the editor had the pleasure of passing a few days, in company with that gentleman, in exploring the geology of the vicinity of New-Haven. Near that town, junctions, on an extensive scale, between widely different formations, are to be observed. A radius of ten miles, with New-Haven for a centre, will describe a circle within which the geological student may find (with the exception of formations, unquestionably volcanic) most of the important rocks of the globe, and a radius of even six or seven miles will include the greater number of these. At, and near the terminations of the primitive ranges, there are rocks which appear to have, in a high degree, the characters of the transition class. Among them is the beautiful green marble of the Milford Hills, seven miles from New-Haven. Mr. Maclure visited that district, and even suggested the first hint which afterward led to the discovery of the marble. Doubts being entertained concerning some of the geological relations of those rocks, a letter was addressed to Mr. Maclure (then in Philadelphia) on the subject. His answer is subjoined. In giving it to the public, the editor takes a liberty which he hopes the respectable author will pardon, because his production, although evidently never intended for the public eye, contains statements and opinions of no small importance to the young geologist, especially of this country. Geology, at the present day, means not a merely theoretical and usually a visionary and baseless speculation, concerning the origin of the globe; but, on the contrary, the _result of actual examination into the nature, structure, and arrangement of the materials of which it is composed_. It is therefore obvious, that the opinions of those men, who, with competent talent and science, have, with a direct reference to this subject, explored many countries, and visited different continents, are entitled to pre-eminent respect. SAUSSURE, by his scientific journeys among the Alps, (although a limited district) has given deserved celebrity to his own name, and, if it were possible, has thrown an additional charm of attraction over those romantic and sublime regions. Dolomieu has made us familiar with the productions and phenomena of volcanoes, those awful and mysterious laboratories of subterranean fire. Humboldt has surveyed the sublimest peaks of both continents, and examined the structure of the globe amidst the valleys of Mexico and the snows of Chimborazo and Pinchinca; and Werner, with opportunities much more limited, (confined indeed to his native country, Saxony) but with astonishing sagacity and perseverance, deduced from what he saw, a classification of the rocks of our globe, which, although not perfect, has done immense service to the science of Geology. In this distinguished group (to which other important names might be added) Mr. Maclure has unquestionably a right to be placed.