102 REPORT UNITED STATES GEOLOGICAL SURVEY. 



tbere were no actual mud eruptions, althougli there may have been one 

 not long before. It was probably in about the same condition as the steam- 

 boat vent in the Gribbon Basin was at the time of our visit this year. In 

 the latter case, however, the steam-vent is in the rock and does not pass 

 through a mud bank as is the case with the Giant's Caldron. 



Grotto. — There is a cave or grotto-like hole in sandstones and clays at 

 the head of a ravine which is a branch of the one leading from the 

 Giant's Caldron. The opening is 3 feet high, 8 feet wide, and about 20 

 feet deep. The top of this entrance is somewhat like a gothic arch, and 

 is coated with a thin coating of vivid green vegetation where the steam 

 strikes the rock. The cavern has a slight downward inclination, and 

 at the back of the cave the steam escapes in pidsations of about 1 minute 

 10 seconds. These pulsations shake the ground, and there is a roaring 

 noise in the cavern. With each pulsation there is a gush of water, 

 which is clear as crystal. The flow from the spring is not very great. 

 In 1871 the temperature was 184° ; in 1872, 182°, and in 1878, 18 lo. This 

 does not necessarily prove a decrease in temperature, as the ditterence of 

 3° may be due to a slight difference in the thermometers or to the 

 difference in the place of taking the temperature. The colors over the 

 entrance to the grotto are green and red, and add to the beauty of the 

 spring. The water has a slight alum taste, and gives off a slight odor of 

 sulphuretted hydrogen. This is the only really clear spring at this 

 locality, and it is due to the fact that the -water escapes through the sand- 

 stones, and escapes from them without coming out into a basin of mud, 

 as do most of the other springs in the surrounding groups. As the 

 accompanying table will show, most of the springs at Mud Volcanoes are 

 either sulphur springs or salses. There are all rrades of the latter. A 

 large number are found on the right bank of the river. They are not 

 included in the table. Professor Hayden visited them in 1871, on his 

 way from the lake to the east side of the falls, and describes them as 

 turbid and mud springs. One he called the mud sulphur spring. It 

 has a basin 15 feet by 30 feet, and in it three centers of ebullition. Pro- 

 fessor Comstock describes one at this locality as follows : 



One of these [mnd bowls], near the waters edge, strongly reminds one of a large 

 kettle of boiling soap ; the resemblance to this uninviting substance being increased 

 by the peculiar streaky appearance arising from the imperfect mixture of the yellow 

 sulphurous, the red or pink ferrnginons, and the blue and greenish argillaceous ingre- 

 dients of the mud. The mud-paste is in a constant state of sputtering ebullition, and 

 it is occasionally spurted with force to a height of 2 or 3 feet. There were some signs 

 about the rim of the bowl of recent more vigourus action. There are some springs 

 bubbling from the river near the northern or eastern shore. The springs are of the 

 same general character as those on the left bank and give off sulphureted hydrogen. 



The following table presents all the principal springs and salses on 

 the left bank, and compares the temperatures of this year (1878) with 

 those of 1871 and 1872. 



The springs are really divided into two groups. Those from l!^o. 1 to 

 No. 12, with ISTos. 24 and 25, forming the first, and those from K"o. 13 to 

 23 the second. 



