Mention should also be made of the profound metamorphism (alteration) of the Cambrian and Ordovician strata along the main axis of the range, where the intense compression, aided by heat and moisture, caused the deeply buried portions of the strata to become plastic, and hence they became more or less foliated (cleavable) and crystallized into various metamorphic rock types, the limestone having changed to marble, the shale to slate or schist, and the sandstone to quartzite. Thus we explain the rocks of the extensive marble quarries of Vermont and western Massachusetts, the slate quarries of central eastern New York, and the Berkshire schist of the Berkshire Hills of Massachusetts. One of the grandest and most significant of all the profound geological processes is the birth and history of a great folded mountain range. Since the Taconic Range affords us such an excellent example of a large-scale, well-understood folded range of great antiquity we may do well to consider it in the light of certain other broad relationships. The great compressive force which folded and upraised the Taconic Mountains did not accomplish its work rapidly in the ordinary human history sense of the word. The force was slowly and irresistibly applied, and the strata well below the surface were gradually bulged or folded, or fractured where near the surface, the length of time required for the operation having been, at the very least calculation, some hundreds of thousands of years, and more than likely a million years or more. Such a length of time is, however, so short compared with all known earth history, that we are accustomed to refer to the formation of such a mountain range as simply an event of geological history.