breccias and agglomerates of angular stones now become _trappean conglomerates_--the stones having been rounded and water-worn--while the fine ingredients, the grit, and sand, and mud, form the rock called _trap tuff_. Fragmental rocks are often quite indurated--the matrix being as hard as the included stones. But as a rule they are less hard than crystalline igneous rocks, and in many cases are loose and crumbling. When a fragmental rock is composed chiefly of rocks belonging to the acidic group, we say it is _felspathic_. When augitic and hornblendic materials predominate, then other terms are used; as, for example, _dolerite tuff_, _greenstone tuff_. STRUCTURE AND ARRANGEMENT OF ROCK-MASSES. 34. The student can hardly learn much about the mineralogical composition of rocks, without at the same time acquiring some knowledge of the manner of their occurrence in nature. We have already briefly described certain sedimentary rocks, such as conglomerate, sandstone, and shale, and have in some measure touched upon their structure as rock-masses. These rocks, as we have seen, are arranged in more or less thick layers or _beds_, which are piled one on the top of the other. Rocks which are so arranged are said to be _stratified_, and are termed _strata_. We may also use the word _stratum_ as an occasional substitute for _bed_. The planes of _bedding_ or _stratification_ are sometimes very close together, in other cases they are wide apart. When the separate beds are very thin, as in the case of shale, it is most usual to term them _laminæ_, and to speak of the _lamination_ of a shale, as distinguished from the _bedding_ of a sandstone. Planes of bedding are generally more strongly marked than planes of lamination. The laminæ frequently cohere, while beds seldom do. In the above figure, which represents a vertical cutting or _section_ through horizontal strata, the planes of lamination are shewn at _l, l, l_, and those of stratification at _s, s, s_. There are hardly any limits to the thickness of a bed--it may range from an inch up to many feet or yards, while _laminæ_ vary in thickness from an inch downwards. [Illustration: Fig. 1.--_st_, sandstone, and _sh_, shale: _s_, lines or planes of bedding; _l_, lines or planes of lamination.] 35. Hitherto we have been considering the _laminæ_ and _strata_ as lying in an approximately horizontal plane. Sometimes, however, the layers of deposition in a single stratum are inclined at various angles to themselves, as in the following figure. This structure is called _false bedding_; the layers or laminæ not coinciding with the planes of stratification. It owes its origin to shifting currents, such as the ebb and flow of the tide, and very often characterises deposits which have been formed in shallow water. (Hillocks of drifting sand frequently shew a similar structure, but their false bedding is, as a rule, much more pronounced.) [Illustration: Fig. 2.--False Bedding.]