E0CKS Of THE ESSEX DEIFI. 



359 



Similar rocks occur in Saxony, where they are known as pyroxene- 

 granulites, in Sweden (hyperite of Tornebohm), in Minnesota 

 (augite-diorite of Streng), near Baltimore (gabbro or hypersthene- 

 gabbro of Williams), and in Scotland, where they have not been 

 described. The original minerals appear to have been augite, 

 hypersthene, magnetite, and plagioclase ; the plagioclase of the 

 Baltimore rocks is bytownite ; in most districts where they occur 

 they show a considerable amount of variability in their mineralogical 

 composition : felspar is sometimes abundant, sometimes absent ; 

 secondary hornblende is frequently present, and sometimes it entirely 

 replaces both the pyroxenes *. 



Crystalline Srhistg. — These rocks are not abundant in the drift. 

 The specimens are numbered 97 to 105 with 171 and ISO. Of these 

 97 to 103 and 180 are hornblende-schists, and are of a greyish- 

 green colour and close-grained. In some cases they are evenly 

 foliated and split with a very level fracture ; in others they are very 

 hard and much contorted, as in no. 100, a section which was taken 

 from a large boulder, exceedingly hard, though rounded and polished. 

 The sections show orthoclase, as a rule, cloudy and much altered, 

 quartz not in any great quantity, and hornblende varying con- 

 siderably. In nos. 97 and 99 it is in long prisms (actinolite), in 87 it 

 is in irregular grains and prisms with good examples of transverse and 

 longitudinal sections and with some instances of twinning. In 101, 

 102 it is not nearly so abundant, but in 103 it is most abundant in 

 grains and prisms so minute that the hand-specimen shows a very 

 compact structure of a silky appearance in which it is hardly 

 possible to detect the separate cystals. But no. 98 is the most 

 interesting of these sections ; for besides the hornblende and some 

 irregular grains of a colonrless mineral showing weak tints under 

 crossed nicols (zoisite ? ), it also contains abundance of dark blue 

 tourmaline (indicolite), the prisms sometimes showing a tendency 

 to gather into radiate or fan-shaped groups. The fact of the 

 occurrence of tourmaline in this rock, when considered in connexion 

 with the number of quartz-tourmaline rocks in the drift, makes it at 

 any rate probable that they have come from the same locality, viz. 

 from some locality where tourmaline-bearing granite is intrusive in 

 hornblende-schists. Xos. 104, 105, and 171 are mica-schists, 104 

 being granariferous : this section was from a block of considerable 

 size, and the garnets are fairly abundant, though much cracked and 

 broken. 



Quartzites and Quartz-Rods. — These rocks are most abundant in 

 the drift ; the quartzites occur in very large boulders as well as in 

 numberless rolled pebbles. Sometimes they are very clearly 

 banded, but in most cases they show no signs of banding. They are 

 usually very fine close-grained rocks with a highly lustrous 

 fracture. Quartz-rocks occur mostly as large, rounded, and highly 

 polished boulders, often, especially in smaller pebbles, of a rose- 

 red colour. The only specimens I have of these rocks are 



* Lehmann "Die Entstehun? der altkrvstallinische Gresteine." Streng. 

 Jfeues Jahrbush, 1877. Williams. BulL TJ. S". GeoL Survey., no. 28, 1886. 



