On the Constituents of Primary Rocks. 309 



mica, and in general, granites are distinguished by having a much 

 greater proportion of felspar than of either of the other two 

 minerals. Sometimes the felspar is formed into well defined 

 crystals, either white or red, it is then called a porphyritic 

 granite. The quartz of such rocks is usually of a glassy lustre, 

 and in very irregular shaped grains. The mica is disseminated 

 in it, in small blackish or silvery scales. Granite rocks of this 

 character, although they pass gradually into gneiss, differ re- 

 markably from it in one particular, all granite being massive. 



When the predominating mineral of the granite, felspar, de- 

 creases very much, and the mica greatly increases, and its innu- 

 merable plates become formed into well defined parallel layers, 

 then granite losing its massive structure, splits in the direction of 

 the mica, and becomes a true gneiss, recognizable by the eye 

 by the parallel lines it externally bears. Students in geology 

 will also observe, that the granite we have been describing, is 

 always found subjacent to the gneiss, and indeed, from no other 

 rock being found inferior to it, granite is considered as the basis 

 of all the primary rocks ; and gneiss, from the constancy with 

 which it is found reposing upon the granite, is considered the next 

 in order of succession. When the principal constituent parts of 

 gneiss, quartz and mica, are finely combined together, and have 

 a yellowish or greenish lustre, then they form a rock which splits 

 into tables easily, and is called mica slate. Sometimes the plates 

 of mica in this rock are larger, and then they form a mica slate 

 of a coarser character. Mica slate is the third rock in the order 

 of succession. There are other rocks in this marine part of the 

 geological limit, occasionally found subordinate to the three 

 members of the primary rocks we have enumerated ; these are 

 principally the hornblende, serpentine, and that calcareous for- 

 mation usually called primitive marble. As the gneiss, which is 

 the base of the city of New York, re-appears across the Sound 

 on Long Island, so the serpentine, which is found massive at Ho- 

 boken, on the Jersey shore, re-appears on the east side of the 

 river not far from the city of New York. 



At Philadelphia, we find this line of primary rocks inflecting 

 inwards from the coast. Near the public Water-works, a well 

 defined gneiss — not different from that at the city of New York — 

 is quarried extensively for foundations of houses. Associated with 

 this, is the Hornblende, which appears close to the Water- works, 



