Crosby. ]j : 52 [October 2, 
that coincide with the bedding. The quartzite sometime appears little 
different from an indurated sandstone; at other points it passes, 
through the increase of mica, into distinct mica slate, which is, per- 
haps, the prevailing rock of this series. ‘The mica slate is never 
very micaceous, and, as in the quartzite, the mica is not uniformly 
diffused through the mass, but is concentrated between the quartzose 
Jaminz, which are quite thin and more schistose than in the preced- 
ing rock. The mica is always in a finely divided, and probably in 
many cases an incipient, state. In some beds, as noticed by Mr. 
Wall, the flakes of mica seem to enclose pellicles of graphitic mat- 
ter. The most interesting rock of this division is a finely schistose 
greenish rock, which, having a base of quartz with, possibly, some 
feldspar, contains a large amount of an unctuous, green mineral, 
somewhat resembling chlorite at first view, but which is probably a 
hydro-mica. It is a massive, thick-bedded rock.  Interstratified 
with all the preceding, and showing occasional transitions into the 
mica slate, are several varieties of a distinct argillite. ‘This rock is 
finely laminar, or fissile and shaly, rarely massive. It is dark-colored, 
usually black, appearing to be always more or less carbonaceous, 
and occasionally distinctly graphitic. The partings are lustrous, and 
are not infrequently marked by a true mica. The accessory miner- 
als in the foregoing rocks are limited to pyrite (occasionally chaico- 
pyrite), quartz, gypsum, and calcite; the last two being quite rare. 
The pyrite and quartz are found in all the rocks deseribed, but most 
abundantly in the argillite, and least so in the quartzite. Occasional 
beds are so highly pyritiferous as, on the oxidation of that mineral to 
assume a decidedly ferruginous aspect. The accessory quartz is in 
the form of lenticular veins, which are for the most part, but not 
always, conformable with the stratification. These veins have been 
well described and faithfully Celineated by Messrs. Wall and Saw- 
kins. They are very numerous in the argillite, and constitute a 
characteristic and important feature of the entire series. 
2. Crystalline limestone in massive beds having in the aggregate 
a oreat thickness, varying in color from white to nearly black, some- 
times decidedly ferruginous, and often with scattered spangles of sil- 
very white mica. ‘The limestones on Chacachacare appear to belong 
here. . 
3. Frequent alternations of the following rocks: argillites simi- 
lar to those in the first series, frequently graphitic and often passing 
into mica slate, the latter rock being less distinctly micaceous than be- 
