244 PROCEEDINGS OF THE ACADEMY OP [1884. 



to merge into a massive rock of the same composition. When 

 the proportion of hornblende increases, it becomes so great as 

 to make up, apparently, the entire composition, in which case the 

 syenitic gneiss runs into the hornblende schist or into massive 

 hornblende rock. The bluish gray trap may be described as 

 follows : in color it A^aries from a light to a dark bluish gray ; in 

 texture, from a coarsely crystalline to one fine-grained, homo- 

 geneous and trappean in character. 



Lithologically, the rock is composed of plagioclase, feldspar 

 and hornblende, with frequently a small proportion of blue 

 quartz and biotite. 



Massive hypersthene has often been found in fine orthorhombic 

 crystals, entirely replacing the hornblende, and is associated with 

 a plagioclase showing the most eminent striation. Thus, from 

 microscopical examination, the rock seems to range from a quartz 

 diorite to a true hyperite^ although no true knowledge of the 

 rock can be had until a thorough microscopical stud}^ is made. 

 Professor G. H. Williams,^ of Johns Hopkins University, has 

 proven a similar rock, in the vicinity of Baltimore, to be a 

 hypersthene gra66ro, which also runs by indistinct stages into a true 

 amphiholite schist. He has also shown that the amphibolite is the 

 result of a paragenesis taking place in the gabbro, the hypersthene 

 and pyroxene found in the latter being altered to hornblende, and 

 thus producing the gradual passage of gabbro into amphibolite. 

 Whether some such alteration as this can account for the passage 

 of the bluish gray trap into amphibolite schist is quite within the 

 range of possibility. This question has already become a subject 

 of iuA^estigation by the writer, and it is hoped that much light 

 will be thrown upon it. 



Calcareous Bocks. — To this class belong those rocks generally 

 called crystalline limestone, of which there are two varieties, 

 namely, saccharoidal limestone and marble. They are found at 

 three known localities, one near Pleasant Hill, one at Hockessin 

 and another near CenJ^eville, both varieties occurring together. 



The marble may be described as very coarsely crystalline. It 

 is very compact in texture, of superior hardness, and is always 

 heavily and closely bedded. The variet}" called saccharoidal 

 limestone is more granular and extremely friable. It is also 



^ Am. Jour, of Sci., October, 1884, and Johns Hopkins Univ. Circular, 

 April, 1884. 



