OF HERTFORDSHIRE BOULDERS. 
57 
EXPLANATION OF THE PLATES. 
Plate III. 
1. (Ashwell, 186).—Section of dolerite showing long prisms of felspar , 
small needles of apatite , and irregular masses of augite. 
2. (Ashwell, 123).—Section of dolerite showing large crystals of olivine , 
traversed by fissures, opaque magnetite , prismatic crystals of felspar , 
and irregular masses of augite. 
3. (Ashwell, 51).—Section of dolerite. The centre shows incipient altera¬ 
tion of felspar along interlacing fibrous lines; crystals of magnetite , 
plagioclase, and augite surround the central mass. 
4. (Ashwell, 124). — Section of basalt showing transparent crystals of felspar , 
black specks of magnetite , and a granular matrix. 
5. (Ashwell, 11).—Section of basalt showing semi-porphyritic crystals of 
olivine , small prisms of felspar, and a few isolated augite crystals. 
6. (Ashwell, 25).—Section of basalt showing large crystals of olivine altered 
along the lines of fissure. 
Plate IY. 
1. (Ashwell, 125).—Section of fine-grained basalt with transparent masses 
of olivine. 
2. (Ashwell, 27).—Section of basalt showing large crystals of altered 
olivine . The small black crystals are magnetite ; the transparent 
prisms are plagioclase enclosed in a glassy matrix with many microliths. 
3. (Ashwell, 66).—Section of porphyrite showing large crystals of plagio¬ 
clase with augite pseudomorphs in a granular matrix. 
4. (Ashwell, 141).—Section of porphyrite showing porphyritic felspar 
crystals in a granular matrix. 
5. (Ashwell, 122).—Section of porphyrite (highly magnified) showing the 
matrix with scales of chlorite and radiating nests of actinolite. 
6. (Ashwell, 72).—-Section of granite showing transparent masses of quartz 
(with fluid and other enclosures), cloudy felspar , and hiotite (with en¬ 
closures). 
Plate Y. 
1. (Ashwell, 33).—Section of granite showing abundance of quartz granules 
and some opaque magnetite and cloudy felspar. 
2. (Ashwell, 30).—Section of granite. The transparent granules are quartz , 
the crystals with strong parallel cleavage are muscovite ; black patches 
of magnetite and cloudy felspar are also seen. 
3. (Sandon, 3).—Section of hornhlendic granite showing masses of quartz 
(transparent), magnetite (opaque), felspar (striated), and a large horn¬ 
blende crystal (near the centre). 
4. (Sandon, 4).—Section of gneiss showing black streaks of magnetite , 
transparent quartz , and striated crystals of mica. 
5. (Hitchin, 3.)—Section of gneiss showing quartz (transparent), mica 
(striated), and felspar (cloudy). 
6. (Ashwell, 150).—Section of schist showing bundles and radiating groups 
of actinolite crystals in a matrix of quartz granules. 
