544 REPORT OF NATIONAL MUSEUM, 1890. 



eruptive rock. This is especially true with the calcareous rocks, 

 and in the specimens exhibited such transitions are sometimes 

 apparent. In the siliceous rocks these transitional forms, are less 

 readily traced. In specimen No. 36018, from Deer Isle, Maine, the con- 

 glomerate character of the rock is made apparent by the presence of 

 pebbles, more or less changed and distorted, it is true, but still unmis- 

 takable indications of the former f ragmen tal character of the beds. 

 The finer grained portions, the material which formed the cementing 

 or binding matter to hold the larger pebbles, yielded, as a matter of 

 course, most readily to the metamorphosiug influences, and gave rise to 

 a fine aggregate of mica scales and other minerals, while the larger 

 pebbles of quartz and feldspars offering greater resistance, have in 

 some instances retained enough of their pebbly characteristics to still 

 be recognizable. Accordingly as they vary in structure, we may divide 

 these metamorphic rocks into two general groups, as below : A, strat- 

 ified or bedded ; B, foliated or schistose. 



A. — STRATIFIED OR BEDDED. 



The Crystalline Limestones and Dolomites. 



Mineral composition. — The essential constituent of the crystalline 

 limestones is the mineral calcite. The common accessories are minerals 

 of the mica, amphibole, or pyroxeue group, and frequently sphene, 

 tourmaline, garnets, vesuvianite, apatite, pyrite, graphite, etc. 



Chemical composition. — As may be inferred from the mineral compo- 

 sition these rocks, when pure, consist only of calcium carbonate. They 

 are, however, rarely if ever found in a state of absolute purity, but 

 show more or less magnesia, alumina and other constituents of the 

 accessory minerals. 



Structure. — The limestones are eminently straified rocks, though this 

 peculiarity is not always sufficiently marked to be seen in the hand 

 specimen. The purest and finest crystalline varieties often show a 

 granular texture like that of loaf sugar, and hence are spoken of as 

 saccharoidal limestones. Statuary marble is a good illustration of this 

 type. (Specimen 17345, from Rutland, Vermont.) Under the micro- 

 scope the stone is shown to be made up of small grains, which, having 

 mutually interfered in process of growth, do not possess perfect crys- 

 tal outlines, but are rounded and irregular in outline as shown in the 

 transparency in the window (No. 39074) and in Fig. 93. All grades of 

 texture are common, the coarser forms sometimes showing individual 

 crystals an inch in length. 



Colors. — The color of pure limestone is snow-white as seen in statu- 

 ary marble. Other common colors are pink or reddish, greenish, blue, 

 through all shades of gray to black. The pink and red colors are due 

 to iron oxides, the greenish as a rule to micaceous minerals. (See color 

 series.) 



